nail.h: adjust really_rewind(): POSIX Issue 7 defined a way..
Oh! I was much too loud stating things on "fucking standard I/O"
and "no standard way to do this"!
Instead, POSIX Issue 7 overloaded the fflush(3) function:
[CX] For a stream open for reading, if the file is not already at
EOF, and the file is one capable of seeking, the file offset of
the underlying open file description shall be set to the file
position of the stream, and any characters pushed back onto the
stream by ungetc() or ungetwc() that have not subsequently been
read from the stream shall be discarded (without further changing
the file offset).
So use the hack unless _POSIX_VERSION>=200809L, then do the
standard-defined 'rewind()+fflush()'.
I still hope i'll be able to replace that with our own homebrew
layer somewhen in the future.